The present invention relates to gold-iron alloy electroplating processes, compositions for use therein and gold-iron alloy electrodeposits produced therefrom.
Gold alloy electrodeposits are extensively used for decorative and functional deposits. Gold alloys with copper, cadmium, cobalt, indium, zinc or tin or mixtures thereof are well known. Examples of patent literature giving details of such compositions revealed by searches by the applicants are JP 53-58023 (Matsushita), JP 51-56241 (Citizen Watch), DE 1696087 (OMF), U.S. Pat. No. 3,926,748 (AMP), GB 1445395 (Schering), GB 1375611 (Lea-Ronal), GB 1279141 (Degussa), GB 2151661 (LPW-Chemie), EP 193848 (Emmenegger), U.S. Pat. No. 4,470,886 (OMI), U.S. Pat. No. 2,724,687 (Spreter), JP 57-120686 (Suwa Seikosha), JP 57-120685 (Suwa Seikosha), JP 56-136994 (Nippon Mining), JP 56-105494 (Nippon Mining) and EP 140832 (H.E. Finishing).
An article in Galvanotechnik vol 83 (1992) pp 808-817 and 1180-1184 by F. Simon mentions gold-iron electroplating using cyanide baths. It refers to gold cyanide complex baths containing cobalt, nickel, indium, iron (it is not clear whether these are present together or separately) in a weak acid bath at pH 3-6.
A search by the UK Patent Office revealed the following cases: GB2242200 (Enthone); GB 1426849 (Deutsche Gold und Silber); EP-A-0480876 (Metaux Precieux); EP-A-0037534 (Degussa); U.S. Pat. No. 4,687,557 (Emmenegger); U.S. Pat. No. 4,358,351 (Degussa); JP-7018484 (Seiko); and U.S. Pat. No. 4,075,065 (Handy and Harman).
Gold-iron baths have the advantage of not inducing allergic reactions in contact with skin such as can be caused by gold alloys containing nickel or cobalt, and do not contain cadmium which is a toxic metal.
It is very desirable to use gold alloy electrodeposits which do not contain nickel or cobalt for skin contacting products, such as rings and spectacle frames.
Gold-iron alloy electrodeposits however are thought to be brittle and to be liable to crack damaging the corrosion resistance of the product. In addition they tend to be too warm a yellow for decorative uses and a paler colour is desired. Colour for gold alloy electrodeposits can be assessed on the (NIHS 03-50) standards scale. NIHS is Normes de l""industrie horlogere Suisse or Swiss watch industry standards. This provides a colour scale ranging from 5N (red), via 4N (pink) to 3N, which is the too warm yellow colour of conventional gold-iron alloy electrodeposits, 2N-18 to 1N 14. The colours are made from gold-silver-copper alloys containing the following amounts for the relevant colours.
The NIHS 03-50 standard states that for gold articles the colour 1N-14 is not obtainable for an alloy of more than 14 carats and for the colour 2N-18 for an alloy of more than 18 carats.
It is desired to produce a gold-iron alloy electrodeposit which has a colour of preferably 2N-18 to 1N-14 on the NIHS scale and which is free of cobalt, cadmium and nickel, and which has good corrosion resistance.
The applicants conducted extensive research to modify the colour of conventional gold-iron alloy deposits. These deposits contain 2.1% iron, 97.9% gold and have a colour of 3N(+).
Addition of zinc sulphate at from 50-200 mg/l gave a colour of 3N to 3N(+); at 300 mg/l the colour becomes too yellow-gray.
Addition of ammonium monovanadate at from 100 mg/l to 1500 mg/l only gave a colour of 3N.
Addition of cadmium acetate on its own or with diethylene triamine penta-acetic acid (DTPA) chelate only gave a colour of 3N.
Lead acted as a metallic impurity, only brown and matt deposits being produced.
Addition of vanadium (IV) oxidesulphate in amounts up to 150 mg/l only gave a colour of 3N to 3N(+).
Addition of ammonium bismuth citrate with DTPA only gave a colour of 3N to 3N(+).
Addition of sodium tungstate dihydrate at from 0.55 to 4.45 g/l of tungsten at current densities of 1 to 4 A/dm2 and at pH values from 3.5 to 4.45 only gave a colour of 3N.
Addition of 5 g/l of nicotinic acid allowed one to increase the current density to 4 A/dm2 without burnt deposits but the colour remained at 3N(+).
Bismuth and lead both acted as a metallic impurity and only brown and matt deposits were produced. Lead was added as lead nitrate. Bismuth was added as bismuth III nitrate pentahydrate.
Addition of potassium stannate 1 g/l at current densities of 1 to 3 A/dm2 only gave a colour of 3N(+).
Addition of cerium (III) nitrate hexahydrate at 1 g/l gave a colour of between 3N and 2N-18. Cerium (III) sulphate, cesium nitrate and cesium sulphate all had no effect on the colour of the deposit.
The applicants then tried addition of zirconium sulphate at 1 g/l at a current density of 1 A/dm2 at 32xc2x0 C. and a pH of 3.14. This gave a deposit with a colour near 2N-18 but very slightly more grey.
EP-A-0193848 is concerned with gold-copper-cadmium-zinc cyanide baths and refers to a number of inorganic brighteners. Baths B1 to B5 show the use of selenium as sodium selenite, arsenic as sodium arsenite and zirconium as the sodium zirconium hydroxy ethyl-imino-diacetate, as inorganic brighteners in B2-B5, no brighteners being used in B1.
Col. 13 l. 38-42 of EP-A-0193848 states that all these deposits are pale yellow and give a colour of approximately 1N-14. There is no teaching of any effect on colour produced by the presence of zirconium. Bath B2 contains zirconium as the inorganic brightener, bath B1 does not contain an inorganic brightener.
In addition it is extremely difficult to obtain a constant colour in the range 1N-14 to 2N-18 with gold-copper-cadmium or gold-copper-cadmium-zinc systems.
According to the present invention an electrodeposit is provided which contains about 1.25 to 1.55% w/w iron, about 1 to 2 ppm zirconium; and about 97.7 to 98.2% gold and has a pale yellow colour less yellow than 3N on the NIHS scale, and preferably at or near 2N-18.
It will be recognised that such a deposit is also of high carat. It is preferred that the deposit be of about 23-23.6 carat.
The gold-iron-zirconium deposits of the present invention are free of toxic and allergy causing ingredients, have high carat values and corrosion resistance and at the same time a desirable pale yellow colour.
The invention also extends to an electroplating bath, free of cobalt, cadmium or nickel comprising gold, as cyanide, iron as a soluble salt or complex, a soluble zirconium salt or complex, a citrate, a weak acid, and optionally a heterocyclic sulphonate such as PPS. The function of the PPS is to allow higher cathodic current densities and to improve the macrodistribution a little.
The gold is preferably present as gold potassium cyanide preferably in an amount of about 1.0 to 10 g/l especially about 2.5 to 3.5 g/l of gold.
The iron is preferably present as a nitrate which may be hydrated. It is preferably present in an amount up to 5 g/l of iron e.g., about 0.1 to 5 g/l preferably about 0.2 to 3 g/l especially about 0.6 to 0.8 g/l. Different contents of iron in the plating bath do not affect the colour of the deposit significantly, but the more iron there is in the bath the more there is in the deposit. However at a current density of about 0.5 A/dm2 as the iron content of the bath increases from about 0.25 g/l, at which the cathodic efficiency is 25 mg/A.min, to 2.0 g/l the cathodic efficiency falls to 7 mg/A.min.
Examples of other salts which may be used instead of iron nitrate are iron sulphate, iron (III) chloride, iron (III) citrate and iron (III) phosphate.
The zirconium is preferably present as the nitrate, which may be hydrated, or less conveniently as the sulphate or as ammonium zirconium citrate complex. The zirconium is preferably present in an amount of about 0.01 to 2 g/l of zirconium e.g. about 0.04 to 1.5 g/l or about 0.1 to 1 g/l, especially about 0.2 to 0.5 g/l.
The citrate is preferably diammonium hydrogen citrate (C6H14N2O7) or (NH4)2C6H6O7 and is preferably present in an amount of about 10 to 500 g/l e.g. about 50 to 200 g/l especially about 75 to 125 g/l. Diammonium hydrogen citrate is preferred to sodium or potassium citrate because it gives much higher macrodistribution of the gold layer e.g. as high as 90% as shown by tests in a Haring cell, as compared with about 50% when sodium or potassium citrate is used.
The weak acid is preferably a hydroxy carboxylic acid such as citric acid (HO(COOH)(CH2COOH)2.H2O, though other carboxylic acids such as oxalic, lactic, formic, thiomalic, gluconic, tartaric, acetic or malic acid could be used. Phosphoric acid could also be used instead of citric acid.
The weak acid is preferably present in an amount of about 1 to 500 g/l e.g. about 10 to 200 g/l e.g. about 20 to 100 g/l especially about 40 to 80 g/l.
The PPS is 3-(1-pyridino)-1-propane sulphonate (C8H11NO3S). It is preferably present in an amount of about 0.1 to 10 g/l e.g., about 0.5 to 5 g/l especially about 1 to 3 g/l.
Materials which can be used instead of PPS include for example pyridine-4-ethanesulphonic acid.
The bath can be used to plate gold-iron-zirconium deposits directly on a range of substrates such as nickel undercoat, or one of the following when provided with a flash of pure gold, namely copper, palladium, palladium-nickel, palladium-cobalt, gold-silver or gold-copper-cadmium.